The instant invention has general applicability to any type of apparatus which requires the introduction of high power, microwave energy from a source, such as a waveguide or antenna, maintained at substantially atmospheric pressure, into the interior of a vacuum chamber, maintained at sub-atmospheric pressure. The microwave energy is preferably introduced into the vacuum chamber for effecting a glow discharge plasma, which plasma is utilized to either deposit a semiconductor or insulating material onto the exposed surface of a substrate or to remove (etch) material from that exposed surface. Whereas, the instant invention has universal applicability to microwave apparatus, said invention enjoys particularly important applicability in the fabrication of photoresponsive alloys and devices for various photoconductive applications, including the fabrication of electrophotograhic photoreceptors.
Since the deposition of relatively thick films of amorphous silicon alloy material and germanium alloy material onto the circumferential surface of cylindrically-shaped drums for fabricating electrophotographic photoreceptors provides the first preferred embodiment of the invention disclosed herein, the instant inventors will primarily discuss the deposition of such amorphous silicon alloy material and amorphous germanium alloy material; however, it is to be borne in mind that the applicability of the high power dielectric window assembly of the instant invention to the deposition of any thin or thick film material is well within the scope of the instant invention. In fact, the microwave glow discharge deposition of many different types of materials, such as thin film or thick film dielectric material or thin film or thick film layers of clear, transparent wear resistant coatings, interference filters, transparent electrically conductive coatings, etc., are also within the scope of the instant invention. Alternatively, and of equal importance, is the fact that the high power microwave window apparatus of the instant invention may be employed with equal advantage in a vacuum chamber adapted to etch or otherwise treat or modify the surface of a substrate.
It must therefore be appreciated that regardless of the type of microwave plasma operation (deposition or etch) being conducted, the rate at which that operation occurs can be controlled, inter alia, by controlling the power at which the microwave energy is transmitted into the interior of the vaccuum chamber. In order to deposit or etch at a high rate, it is necessary to utilize high power levels, e.g., in the kilowatt range and preferably 3 or more kilowatts. The trouble which arises from the use of high power microwave energy is that said high power microwave energy tends to cause heating of the dielectric window through which said microwave energy is coupled into the interior of the vacuum chamber. Prolonged or excessive heating of the dielectric window can cause the cracking thereof, which cracking results in the catastrophic failure of the deposition/etch operation. Of course, because microwave plasmas are highly energetic in nature, even the introduction of relatively low microwave power into the vacuum chamber over a relatively lengthy period of time can also cause the dielectric window to overheat and fail. Therefore, there exists a need for a dielectric window through which high power microwave energy can be coupled into the interior of a glow discharge plasma deposition/etch chamber, which dielectric window is capable of prolonged usage without failure.
As mentioned hereinabove, the instant invention has particular relevance to the fabrication of electrophotographic photoreceptors because the semiconductor alloy material required to be deposited upon the circumferential surfaces thereof can be over 40 microns in total thickness. (Note that this is to be contrasted with the fabrication of thin film solar cells which only require the deposition of less than 1 micron, in total thickness, of semiconductor alloy material.) The relevance of the instant invention to electrophotographic drums is because the economics of fabrication necessitate that a high deposition rate process be employed. Due to the particular relevance of electrophotographic photoreceptors to the instant invention, the following paragraphs are intended to provide a better understanding of the structure of said electrophotographic photoreceptors in which it is contemplated that said microwave deposition apparatus will be initially utilized.
Approxiately 45 years ago, C. Carlson developed the first electrophotographic process based upon a sulfur material. Other chalcogenides such as selenium and selenium alloys were thereafter suggested for use in such applications together with organic substances such as polyvinyl carbazole (PVK). Selenium and selenium alloys however, were found to possess several inherent shortcomings including for example, high toxicity, which renders the drums difficult to handle; relative softness making said materials subject to rapid wear and abrasion; and poor photoresponsiveness, particularly in the infrared region. In contrast thereto, amorphous silicon alloy materials were considered practical alternatives because they were found to be relatively hard, non-toxic and able to demonstrate excellent photoresponse to infrared radiation. Also, by this point in time, it was possible to fabricate amorphous silicon alloy materials with a reduced density of states so that charging of those materials to the potentials required for electrophotographic replication was considered possible. Thus, it was realized that photoreceptors formed from amorphous silicon alloy material, if such photoreceptors were manufacturable in an economical fashion, would provide superior environmental, photoresponsive and structural characteristics, vis-a-vis chalcogenide photoreceptors.
With the passage of time, research into the fabrication of amorphous silicon alloy materials continued and the density of localized states in the energy gap thereof were further reduced; and hence, the quality of those materials for all photoresponsive applications were improved. These materials of improved quality were preferentially deposited by a glow discharge decomposition process wherein a silicon containing feedstock gas such as silane was introduced into a vacuum vessel. It was within said vessel that said feedstock gas was decomposed by an r.f. glow discharge and deposited onto the surface of a substrate at a substrate temperature of about 225 to 325 degrees Centigrade and a pressure of about 0.5 torr. The semiconductor alloy material so deposited was an intrinsic (although slightly n-type) amorphous silicon alloy material consisting of silicon and hydrogen.
In order to produce a doped amorphous silicon alloy material, a gas containing a Group VB element such as phosphine or a gas containing a Group IIIB element such as diborane, was premixed with the feedstock silane gas and passed through said glow discharge vacuum vessel under the same operating conditions as set forth in the previous paragraph. By employing these dopant gases, it became possible to fabricate layers of either n-type or p-type amorphous silicon alloy materials. The fabrication of amorphous silicon alloy material in this manner combined hydrogen with silane at an optimum temperature so that the hydrogen was able to passivate some of the dangling, strained or otherwise stressed bonds of the deposited silicon matrix material, thereby substantially reducing the density of localized states in the energy gap thereof. The result was that the electronic and optical properties of the amorphous silicon alloy material were vastly improved.
While the amorphous silicon alloy materials made by the process described hereinabove, demonstrated photoresponsive characteristics suitable for the production of photovoltaic devices and other photoresponsive applications, any type of process which relies on r.f. generated plasmas suffers from relatively slow deposition rates and relatively low utilization of feedstock gas. Both of these deficiencies are important considerations from the standpoint of the commercial manufacture of photovoltaic devices and, particularly, to the commerical manufacture of electrophotographic photoreceptors. Indeed, by employing r.f. glow discharge processes, it was only possible to obtain a deposition rate of less than about 20 angstroms per second and the production of a single electrophotographic drum required approximately 24 hours. Additionally, these prior art r.f. processes which increased the magnitude of the power density in order to obtain enhanced deposition rates, resulted in the production of films having poor electrical properties due to an increased density of defect states in the deposited silicon alloy material. Further, said prior art r.f. processes were inherently limited in the degree to which the feedstock gases introduced into the vacuum chamber could be energized, and hence the rate of deposition which could be achieved.
As the inherent advantages of amorphous silicon electrophotographic photoreceptors and the inherent shortcomings of the r.f. glow discharge fabrication of those photoreceptors became apparent, the assignee of the instant invention undertook research directed toward the development of a faster, more economical and more efficient method of fabricating amorphous silicon alloy materials for use in electrophotographic applications. Such a method, which includes the employment of a refreshingly innovative apparatus for the simultaneous deposition of silicon alloy material onto the circumferential surface of a plurality of electrophotographic photoreceptors was developed and is fully described in commonly assigned U.S. Pat. No. 4,729,341 to Fournier, et al for "Method and Apparatus for Making Electrophotographic Devices", the disclosure of which is incorporated herein by reference.
The specification of the Fournier, et al reference teaches the construction of an apparatus specifically adapted to utilize microwave energy so as to facilitate the simultaneous, uniform, microwave glow discharge deposition of amorphous silicon alloy material over the entire circumferential surface of a plurality of elongated, substantially cylindrically shaped drum members. Those drum members have successive layers of silicon alloy of differing conductivity types or differing amorphicity deposited thereupon so as to be used as the photoconductive media for electrophotographic copier machines. By utilizing the concept of microwave initiated glow discharge taught by the Fournier, et al '341 reference, substantially all reaction feedstock gas introduced into the vacuum chamber is decomposed. Further, by utilizing the special geometry defined therein by the aligned, spacedly positioned, cylindrically-shaped drum members, over 70% of the decomposed reaction gases may be uniformly, simultaneously and rapidly deposited upon the circumferential surfaces of those cylindrically shaped drum members. Therefore, both, the feedstock gas conversion efficiency and the utilization efficiency is extremely high, vis-a-vis, comparable r.f. plasma apparatus.
The structural arrangement of the elements in that microwave deposition apparatus must be understood in order to understand the manner in which the instant invention defines thereover. The microwave deposition apparatus of Fournier, et al '341 includes a substantially enclosed inner chamber defined by the aforementioned plurality of closely spaced, operatively disposed, cylindrically shaped members. The inner chamber includes a plasma deposition region into which feedstock reaction gas is introduced. The feedstock gas is decomposed by microwave energy also introduced into said plasma deposition region by a waveguide through an alumina window assembly. The alumina window assembly comprises a single, planar alumina window permanently affixed to the terminal end of said waveguide and disposed in operative communication with said inner chamber. The alumina window not only defines one end of the plasma region, but said window also forms the vacuum seal between the waveguide (maintained at atmospheric pressure) and the sub-atmospheric chamber. It is this arrangement of apparatus which efficiently transmits relatively, (vis-a-vis, the kilowatt power ranges now being investigated) low power microwave energy into the plasma region of the vacuum chamber for effecting the deposition of decomposed gases onto the circumferential surfaces of the photoreceptors.
At relatively low levels of microwave power, the microwave deposition apparatus of Fournier, et al '341 is adapted to deposit, for example, approximately 50-100 angstroms per second of amorphous silicon alloy material onto the circumferential surface of the cylindrically shaped members. While this deposition rate represents a significant improvement over the deposition rate achieved by conventional r.f. glow discharge methods (as well as the concommitant improvement in feedstock gas utilization), if the power density of microwave energy being introduced could be further increased; (1) still more efficient gas decomposition and hence deposition rates could be obtained and (2) the deposition of microcrystalline silicon alloy material would be simplified. Obviously, such higher power densities would additionally provide for increased and more efficient etching processing in applicable situations.
The inventors of the instant invention have attempted to improve the efficiency of deposition of the silicon alloy material in such drum deposition apparatus by increasing the microwave power level so as to deposit said silicon alloy material at a rate in excess of approximately 100 angstroms per second. This method has indeed proven successful in increasing deposition rates and in facilitating the economical deposition of microcrystalline silicon alloy material; however, the increased power densitites have exposed a weakness in the design of that microwave initiated glow discharge deposition apparatus. Specifically, the alumina window of the Fournier, et al '341 microwave deposition apparatus was proven to be incapable of withstanding the elevated temperatures generated by the more energetic microwave plasma initiated by utilizing high power densities. Moreover, the inventors of the instant invention have observed catastrophic failure, such as rupture and cracks in both the alumina window and the vacuum seal (which seal effects an airtight closure between the waveguide and the alumina window). The instant inventors have also found that similar failure modes of said alumina window develop during lengthy periods of operation of the microwave apparatus at even relatively low power densities. Said inventors are confident that both of these failure modes re a result of overheating of the window occasioned by (1) the failure to properly match the coefficient of thermal expansion of the material from which the dielectric window is fabricated with that of the vacuum seal, and (2) the fact that because alumina is characterized by a relatively low resistance to thermal shock, the dielectric window cannot withstand, for lengthy periods of time, the elevated temperatures, (temperatures in excess of 500.degree. Centigrade) typically associated with high power microwave plasmas. It is to be noted that the typical failure modes of said dielectric window are occasioned by (1) the exposure of the window to elevated temperature; (2) the deposition of amorphous silicon alloy material onto the surface of the window, which material crystallizes due to elevated temperatures, thereby absorbing microwave energy and forming a hot spot on the window.
Thus, it should be appreciated by those skilled in the art that the power densities employed in microwave deposition apparatus have heretofore been limited by the inherent structural ability of the microwave window assembly to withstand the elevated temperatures associated with plasmas generated by high power microwave energy. It has further been determined that while the aforementioned failure modes may be alleviated by forming the window from a different dielectric material, a more permanent solution would be to provide adequate cooling for said window or window assembly. This is because while a different material would elevate the amount of microwave power which could be introduced before that material failed, an adequate cooling scheme would prevent failure at all practical power densities. Of course, the best of both worlds would be to select optimum dielectric materials and provide adequate cooling for the windows fabricated therefrom.
While the aforementioned discussion has dealt with apparatus for the deposition of materials utilizing high power microwave energy, as mentioned hereinabove, the instant invention may also be employed in apparatus adapted to etch or otherwise treat a surface by a high power microwave sustained etchant plasma. Prior art devices which employ radio frequency energy to initiate and sustain plasmas of precursor etchant gases, have proved defficient in providing a sufficient level of plasma intensity and feedstock gas utilization. Due to the defficiencies inherent in r.f. plasmas, increasing interest has been shown in the use of microwave energy to generate and sustain etchant plasmas. Unfortunately, microwave etching apparatus have heretofore employed the same type of single window assembly design described in detail hereinabove with respect to deposition apparatus. Thus, the amount of microwave power which could be employed in such etchant assemblies was limited by the ability of the dielectric window thereof to withstand the elevated temperatures produced by exposure to highly energetic microwave initiated plasmas.
Accordingly, a need exists for an improved window assembly which can efficiently, economically, reliably, and safely transmit relatively high power microwave energy from a waveguide into a vacuum chamber, for both deposition and etch operations, without suffering damage due to prolonged exposure to elevated temperatures.